Heating boiler with heating surface reduced by improved convection



1952 'J. A; RYDBERG 2,619,941

HEATING BOILER WITH HEATING SURFACE REDUCED BY IMPROVED CONVECTION Filed Feb. 1, 1949 4 Sheets-Sheet 1 W: yn; 2

Dec. 2, 1952 J. A. RYDBERG HEATING BO ILER WITH HEATING SURFACE REDUCED BY IMPROVED CONVECTION 4 Sheets-Sheet 2 Filed Feb. 1. 1949 lNl/ENTOR JOHN ANDERS RVDBERG ATTORNEY Dec. 2, 1952 .1. A. RYDBERG HEATING BOILER WITH HEATING SURFACE REDUCED BY IMPROVED CONVECTION 4 Sheets-Sheet 55 Filed Feb. 1, 1949 R m m V m JOHN A/VDE 1% RVDBERQ BY Z 2: MQRNEY Dec. 2, 1952 A. RYDBERG 2,619,941

J. HEATING BOILER WITH HEATING SURFACE REDUCED BY IMPROVED CONVECTION Filed Feb. 1, 1949 4 Sheets-Sheet 4= INVENTOR JOHN ANDERS RVDBERG Patented Dec. 2, 1 952 HEATING BOILER WITH HEATING SURFACE REDUCED BY IMPROVED CONVECTION John Anders Rydberg, Stockholm, Sweden, as-

signor to Aktiebolaget Gustavsbergs Fabriker, Gustavsberg, Sweden, a corporation of Sweden Application February 1, 1949, Serial No. 73,888

' In Sweden January 30, 1948 6 Claims.

This invention relates to a heating boiler, in which by special construction an improved convection is obtained, while at the same time the heating surface required is reduced. This invention is related to that disclosed in application Serial No. 73,890, filed February 1, 1949.

Heating boilers of the type referred to consist of a combustion chamber and a, flue gas system connected with same, in which water-filled partitions connected with the water space of the heating boiler conduct the flue gases in zigzag paths, before they emerge through the flue gas discharge, usually a chimney.

The draft required for a heating boiler is usually generated by means of a chimney. In certain cases, however, it has been suggested that the draft should be improved by means of a fan. Such a fan has also been used for overcoming excessive resistance in the grate or fuel layer. Any increase in the flow rate of the flue gas current in the boilerfiues was not desired beyond the normal gas flow rate resulting from a good chimney and other satisfactory draft conditions, which rate can be stated to be less than W=l.5N/m. s. where W is the gas flow rate in cubic meters per square meter of gas channel flow section per second and the normal gas volume N is expressed as m. gas at C. and 760 mm. H (normal cubic meters).

In heating boilers with a fan it has been found to be economically advantageous to increase the gas flow rate in the lines above the hitherto usual flow rate. At an increasing gas flow rate it will be found that the heat transfer coefficient of the heating surface increases, and thus the size of heating surface required for a certain purpose decreases. The dimensions and space required for the heating boiler can thus be reduced.

It can be proved, however, that if the gas flow rate increases above a certain limit, the cost of power for the fan will increase faster than the cost of the heating surface decreases. This invention is based on the knowledge that a higher gas flow rate is suitable and that such a rate can be determined as the most economical, i. e. a rate, at which the total of operating cost for generating the draft and of the annual amortization for the heating surface required can be determined as a minimum.

The pressure drop P in the heating boiler flues can be expressed by formula:

(1) P=A.w-

The heat transfer coefiicient k for the convection heating surface is approximately:

2 (2) k=B.W"

In these expressions, A, B, n and u are constants, and W: the gas flow rate in the boiler flues in N. The formula for obtaining the pressure drop P given above is based on the fact that such pressure drop is dependent on the gas flow rate W in the flues. Experiments have shown that P is proportional to an exponential function of the gas flow rate W as expressed by the formula where n is the exponential value by which P depends on W and in which -A is a factor of proportionality. As in the formula for k the terms A, B, n and u are numerical values. Those for n and u have been found by experimentation to be 1.8 and 0.8 respectively, and for B to be where d is the hydraulic diameter of the flue.

The total cost of the power for the fan and of the annual amortization for the convection heating surface can be expressed thus:

G=gas volume per time unit =cost of power E=convection heating surface area a=annual amortization per m? heating surface Z=operating time per year.

For cooling down in the boiler flues there is the following relation:

t t, an (4) ar- 0.0,,

Where If it is desired to cool down the flue gases to a certain value, independently of variations in gas flow rate and in heating surface, this can be expressed by:

3 The above expression for the cost K may now be written by substituting (l) (2) and (5) in (3) My investigations have shown that one can put, with sufficient safety, 2.6 for u+n.

This shows that the variation in boiler amortization and power prices, a and T respectively, and the operation time Z has no considerable effect upon the economical gas flow rate We. 1 have thus succeeded in establishing that there is an economical value of gas flow rate and that this value We will vary comparatively slightly in the case of variations in prices and method of operating. My investigations have further shown that We normally is in the neighborhood of fiN/m s. and that this value has comparatively general validity. Around the economical value We for the gas flow rate, cost K is found to vary very slightly. Owing to my investigations I have found that from a practical and economical point of view it is sufficient, if the gas flow rate is kept Within the limits (3 to 1O)N/m. s. As an exampic I show in Fig. l of the drawings a diagram of the relation between the cost per year for cooling down the gas flow in N per operating hour and the gas flow rate W in N /m. s. in the flue system. The gas flow has been assumed to enter the convection surface at 1100 C. and to leave it cooled down to 200 C, and the number of operating days of 24 hours each per year of the heating boiler has been assumed to be 240, all values quite normal. The diagram shows that the cost per year has a minimum at a gas flow rate of 6N/m. s., and that the curve around this value is rather flat, which means that the gas flow rate W can be varied within the limits (3 to 10)N /m. s. without considerably increasing the cost per year.

The invention thus relates to a heating boiler of the type previously described, with a fuel storage compartment and flue system and a fan arranged between the flue systemand the flue gas discharge, for instance a chimney. According to this invention, the flue gas system is constructed as a convectionsurface with a high heat transfer figure by locating the flue gas flow rate within the limits (3 to 10)N/m. s. by means of suitable selection of the capacity of the flue gas fan in relation to the heating surface of the boiler. This invention relates in particular to a heating boiler with a gas flow rate in the flue system of GiN/mfis. or in the neighbourhood of this figure. In the case of large heating boiler plants of, for instance, 1 million kilogram calories per hour for larger buildings or similar purposes, the convection heating surface can be re duced to half the area with good economy. It is evident that by this a considerable saving in space and cost is attained. Moreover, it has become possible, owing to this invention, to exceed the upper limit for heat production in one single boiler unit, which in practice formerly had been fixed with regard to the possibility that they might be placed in, for instance, a block of buildings.

As distinguished from other known heating devices with increased flue gas flow rate, the water spaces of the fiues of the present boiler are constructed as plane water walls.

The construction of the heating boiler proposed may be varied in other respects than those already mentioned, by making use of known principles for heating boilers, in particular the construction of large ones. called to a special design where a secondary combustion or flame chamber is arranged between the fuel storage compartment and the fluesystern.

A heating boiler constructed according to this invention is shown as an example in'Figs. 2-.4 of enclosed drawings. Fig. 1 is a diagram showing the relation between the cost K/G per year for cooling down the gas flow in cubic meters per operating hour andthe gas flow rate .W in meters per second, Fig. 2 is a longitudinal section of the heating boiler, Fig. 3 is a sectional view on the line 3-3 of Fig. 2, and Fig. 4 is an elevational View of the boiler seen from the rear.

Referring to Fig. 2, the heatingboilershown has a fuel compartment a forming a shaft for the fuel, which is put in from the top of the boiler. The lower portion a of compartment 4 provides a primary combustion chamber. The fuelcompartment is closed as shown in Figs. 2 and 4 by means of a removable cover [4 of the following construction. The cover l4 fits tight with a groove l3 against the upper edge 9 of the fuel compartment and can by means of a lever mechanism l2, 2i and bar M. be lifted to a position, where by means of suitable mechanical devices it will rest on two pairs of wheels 1!,722, which in turn are 50 arranged that cover Mwhen lifted is moved to one side on rails consisting of Z- shaped beams It, 23 on the boiler top. The top of the fuel storage compartment will thus'be open for filling it with large size fuel such as wood. The removable cover M in turn has an opening it closed by a smaller lid It, attached by means of a bayonet catch 19 and intended for putting in less bulky fuel, such as for instance coal, etc. The bar i4 is attached to the lid It. The cover unit is fitted with protective plates I8, 2 5 on the side exposed to the fuel compartment as shown in Fig. 2 and with insulation 11 and 2B of glass wool, asbestos or similar material put in between.

Certain of the walls of the fuel compartment and primary combustion chamber are constructed as plane, flat water spaces 5,2S, 3!, 85, 85.

The water spaces in the two walls in longitudinal direction of the boiler, 85, 85,v are enlarged ,to form longitudinal chambers 65, 6, below the boiler top. The return water enters, one of these chambers through a connection 1. Water space 5 at the front of the boiler communicates by means of a connection 8 with the hot water line fed by the boiler. On the boiler front there are two pairs of furnace doors 2, ,3one pair 3 above and the other pair 2 below a removable grate l in the lower part of the fuel shaft. Grate! consists of Attention is, however,

tubes attached between the front and rear water spaces and 3| respectivelyof the fuel compartment and primary combustion chamber. Water flows through the grate tubes 1 so that the grate is water-cooled. In the wall 25,. 3| of the fuel compartment and primary combustion chamber there is a flue gas opening, 30, through which the flue gases pass to subsequent sections of the boiler. Disposed within the fuel storage compartment is a fuel bridge 21, slantingdownward- 1y from a point at the rear of the fuel shaft and terminating at a suitable height above the grate l for instance a little above the level of the upper edge of flue gas opening 30. Said fuel bridge 21 consists of a water-filled wall communicating with the other water spaces of the boiler by means of connections 25, 28. The purpose of fuel bridge 27 is to provide a larger separating area 29 at the gas discharge opening 39 in the lower part of the fuel storage compartment, where the flue gases are discharged. Thus the resistance at the gas discharge opening 3b from the fuel storage compartment is reduced owing to lower gas flow rate in said separating area 29. This is advantageous when small size fuel is used, such as coal, coke, sawdust, or similar material. The boiler also becomes less sensitive to accumulation of ash and slag at the flue gas opening 36, which might cause considerable resistance. r

The flue gases leaving through gas discharge 30 are conveyed to a secondary combustion or flame chamber 4! while being mixed with secondary air entering through a secondary air inlet 32. In this inlet 32 a damper 33 is arranged for regulating the quantity of incoming secondary air. In the secondary combustion chamber ll a thermostat 42 can be arranged, which operates the damper 33. Said secondary combustion chamber 4| is arranged between the rear Vertical water space iii of the fuel storage compartment and a subsequent, likewise vertical water space 40, which water spaces communicate with the longitudinal water spaces 85, 85 of the boiler. That part 3| of the rear wall of the fuel compartment, which lies below flue gas discharge 3B, is water-filled and bent first rearwardly and then upwardly for directing the flue gases into the secondary combustion chamber 4|. This part 3! of the wall forms together with the Vertical water space 46 the limiting surfaces ary air intake 32. The flue gases are forced to flow upwardly to the secondary combustion chamber il through a substantially S-shaped conduit 31, formed by horizontal ledges 36 and 38, projecting into the secondary combustion chamber 4| from walls 26 and 46 respectively. These ledges 36 and 38 are also water-filled and are intended for mixing the flue gases with incoming secondary air. It has provedvery diflicult to mix the flue gases and the secondary air. The secondary air intake 32 enters in the lower part of the S-shaped mixing conduit 31 through a number of oblique slits 34, parallel with the longitudinal direction of the boiler, in order to divide. the secondary air into a number of parallel air currents. The flue gas and secondary air currents by this device are forced to thoroughly mix with each other, and mixing is completed at the points of change in direction, which follow immediately, as stated above. By these changes in direction a violent whirling motion of the gas mixture is obtained, so that complete combustion results with a minimum surplus of air.

If desired the limiting edges surrounding the of the secondsecondary air inlet slits 34 may extend upwardly in the shape of guide sleeves so that these sleeves terminate approximately at the level of the lower edge of the lower horizontal ledge 36. This will also divide the flue gas flow into a plurality of paths which facilitate the efiicient mixing with secondary air.

The temperature in the'secondary combustion chamber 4] varies with the load and surplus of air in such a way that the heavier the load, the higher the temperature in the secondary combustion chamber. Further, the larger the surplus of air, the lower the temperature in the secondary combustion chamber at the same load. The demand for secondary air is larger at heavy loads than at light loads and it is smaller at a large surplus of air than at a smaller one. By placing in the secondary combustion chamber it the thermostat 42 the supply of secondary air may be controlled by means of damper 33 operated by the thermostat depending upon whether the temperature of the secondary combustion chamber drops or rises. In the case of hollow spaces in the fire, for instance, a considerable surplus of primary air and a low temperature in the combustion chamber is obtained, which is accompanied by a decrease in secondary air supply. If a slide occurs in the fire, there will be a sudden increase in combustion intensity with a great demand for secondary air. The temperature in the secondary combustion chamber rises, while the supply of secondary air increases at the same time.

From the secondary combustion chamber 4| the flue gases enter the convection zone of the heating boiler. This zone is formed by a number of hollow, relatively flat, water-filled, vertical partitions 49, d9, 85, 55 and 11, which alternately project upwardly from the bottom of the boiler and downwardly from the top of the boiler, so that the flue gases flow through the vertical passages, 84, ill, 14, I5, thus formed in zigzag paths before leaving the boiler through the smoke gas exit 73. The flue gases enter the convection zone through opening 44, which is formed at the top in the rear part of the secondary combustion chamber by the rear water-filled wall 40 terminating below the boiler top. The sides of passages 84, 8|, M and 15 facing each other are fitted with vertical flanges or fins 63, which are situated opposite each other in pairs and almost meet, and which divide the flue gas current into parallel partial currents.

An important detail for rendering possible high economical gas flow rates is that the boiler flues 84, SI, 14, 76 are shaped as diifusers 83, 53, 18, El in connection with every change in direction, i. c. When changing from downwardly to upwardly direction and vice versa, so that the gas flow rate is reduced before the direction is changed. This has been attained by making the ends 83, 53, 78, 6| of partitions 46, 59, 8B, 55, i7 tapering at the bends 82, 52,19, to of the flue gas system so that the flue gas passages expand. This reduces the pressure losses in the flue bends :82, 52, 19, 6E to acceptable values. Without this arrangement, the combined pressure drops in the bends would dominate, which is not desirable, as it is only pressure drops due tofriction along the heating surface that efliciently contribute to an improvement of heat transfer.

Partitions dd, 39, 8Q, 56, 'l'i' communicate with the longitudinal water spaces 85, B5 of theheat-. ing boiler. Between a horizontal ledge of wall part 26 and partition 49 there is a soot opening 43 at the top,'which is closed by a removable cover. .46; Soot hole covers t and 51 also close openings 51 and 58 arranged on the boiler top betweenpartitions 39 and 56, and between 56 and 11 respectively. Of these soot hole covers, the former, 46 and 5B, are fitted with a protective plate underneath, it and 55 respectively, and with insulation inserted between these plates and upper plates 41 and 54 respectively, for instance glass wool, whereas the last soot hole cover 51 is of simpler'construction and without insulation. ,On top of the secondary combustion chamber and the convection chambers as shown in Fig. 2 a removable cover plate 15 is placed on suitable supports of sectional iron. Secondary air is assumed to enter through opening 59 left by the cover plate 45 at the rear upper end of the convection system, flowing along the space under the cover'plate and entering vertical passages 39 at the sides of the secondary combustion chamber ii and finally entering, duly preheated, the secondary combustion chamber M through secondary air intake 32.

The flue gases are discharged at the top from the last gas passage 62 of the boiler, entering a vertical flue gas discharge, passage 82, which is connected with a flue gas fan 88, as shown in Fig. 2, by means of a central outlet 6% and. with the chimney by means of a smoke gas discharge 13 at the bottoms Fan 68 is in turn also connected with said smoke gas discharge it. In the smoke gas discharge flue 62 two dampersare arranged: one, 64, upstream of the fan outlet 66, and another one, l5, downstream of the same. The former damper 64 is kept open sufficiently for regulating the discharge ofv flue gas, while the latter, 15, is kept closed in order to prevent closed circulation of the gas volume under the upper damper 64 through fan 68. On fan casing .6! there is arranged a supporting structure 69 :forthe motor-l0, whichby means of a belt. ll

drives shaft 12 of the impeller.

The longitudinal sides and the rear of the heating boiler are covered with a layer of insulating-material 65.

The heating boiler shown is constructedwith regard to such a design of the heating surface and adaption of the fan so that the above explained advantageous gas flow rate in the convection system of "(3 to )N/.m. s.,in.partic ular6N/m. s. or a figure in that neighbourhood, is obtained.

What I claim is:

l. A water heating boiler of the water wall type having a combustion chamber, flue gas passages of substantially rectangular cross-section in said boiler defined by water filled parallel and substantially vertical partitions, fins on said partitions disposed ,in said passages, said partitions alternately terminating in spaced relation: to the upperand lower internal walls of saidboiler thus providing an elongated serpentine path for said gases, afiue gas outlet and a circulating fan disposed between said passages and said outlet, the capacity of said fan and the cross sectional :area and arrangement of said passages being so coordinated that the rate of flow. of said gases within said passages is within the limits (3 to ,10)N/m. s. and the heat transfer coefficient is substantially equal to BW- and in which m. =Cross section of flue in square meters s.=seconds d hydraulic diameter of the fiueand W=gas velocityin meters/sec.

2..A water heating boiler of the water wall type having a combustion chamber, flue gas passages in said boiler defined by water filled parallel and substantially vertical partitions,- said partitions alternately terminating in spaced relation to the upper and lower internal walls of said boiler thus providing an elongated serpentine path for said gases, a flue gas outlet and means disposed between said passages and said outlet for forcibly circulating said gases, the-capacity of said means and the crosssectional area and arrangement of said pasages being so coordinated that the rate of flow of said gases is within the limits (3 to '10) N/mPs. and the heattransfer coefiicient is substantially equal'to BW and in which N=normal gas volume in cubic meters at 0 and 760 mm. Hg

m. =Cr0ss section of flue in square meters s.=seconds d=hydraulic diameter of the flue and W=gas velocity in meters/sec.

3. A heating boiler as defined in claim 1 in having a combustion chamber, flue gas passages in said boiler defined by water filled parallel and substantially vertical partitions, said partitions alternately terminating in spaced relation to the upper and lower internal walls of said boiler thus providing an elongated serpentine path for said gases, a fiue gas outlet and a circulating fan disposed between said passages and said outlet, the capacity of said fan and the cross sectional area and arrangement of said passages being so coordinated that the rate of flow of said gases'within said passage is within the limits (3 to"10)N/m; s. and the heat transfer coefiicient is substantially equal to BW and in which N normal gas volume in cubic meters at 0 and 760 mm. Hg

m. =Cross section of flue-in square meters s seconds d=hydrau1ic diameter of the fine and W =gas velocity in meters/sec.

6. A water heating boiler of the water walltype' having a combustion chamber, flue gas passages in said boiler defined by water filled parallel and substantially vertical partitions, said partitions alternately terminating in spaced relation to the upper and lower internal walls of-said boiler thus providing an elongatedserpentine path for said gases, opposed vertical fins on said partitions providing separate gas flow paths, a flue gas outlet and a circulating fan disposed between said passages and said outlet, the capacity of said fan and the cross sectional area and arrangement of said passages being so coordinated that the rate of flow of said gases within said passages is within the limits (3 to 10) N/mf sand the heat transfer coefiicient is substantially equal to 13W"- and in which N=normal gas volume in cubic meters at 0 and 760 mm. Hg

9 mP=Cross section of flue in square meters Number s.=seconds 1,948,538 B=3.8d-- 2,053,590 d=hydrau1ic diameter of the flue and 2,463,958 W=gas velocity in meters/sec. 5

JOHN ANDERS RYDBERG. Number REFERENCES CITED 5 225 The following references are of record in the 10 97:24; file of this patent: 411,280

UNITED STATES PATENTS Number Name Date 1,198,838 Gurney Sept. 19, 1916 1,210,161 Gorr Dec. 26, 1916 15 1,360,980 Von Der Lippe Nov. 30, 1920 1,724,462 Doherty Aug. 13, 1929 Name Date Noack Feb. 27, 1934 Whiteley Sept. 8, 1936 Fisher Mar. 8, 1949 FOREIGN PATENTS Country Date Great Britain Feb. 9, 1895 Denmark Jan. 8, 1934 Sweden Oct. 31, 1939 France June 13, 1910 OTHER REFERENCES Finding and Stopping Waste in Modern Boiler Rooms, third edition, revised and enlarged, page 530. Cochrane Corporation, Philadelphia, Pa. 

